In the course of operating aircraft, the pilots thereof typically are called upon to react or respond to a number of dynamic conditions which concern phenomena the subject of which has long been analysed under theory of flight principles. It is generally recognized that such pilot activity becomes more intense at the commencement and termination periods of a given flight. For instance, during departures, attention is paid to achieving a best rate of climb, or a rate considered ideal where it is necessary to clear an obstacle near the runway. In this regard, proper aircraft control procedures ultimately stem from such theoretical considerations as airfoil lift, angle of attack and drag. Similarly, desirable landing procedures, particularly under shortfield landing conditions, require maintenance of the aircraft in an ideal attitude with respect to the above parameters.
In each of the above typical control situations, the aircraft is operated much closer to its aerodynamic stall condition than in other, cruise designated phases of flight, thus emphasizing a need for close concentration on the part of the pilot. Piloting an airplane during these critical periods additionally requires added levels of human information processing or attention by virtue of the responsibility of the pilot to react to a much higher frequency of air traffic and navigational information inputs. Accordingly, during the more critical phases, particularly take-offs and landings of any given flight, the concentration capabilities of the pilot are taxed to the highest extent.
Inasmuch as a significant number of fatal general aviation accidents have been found to be occasioned by aerodynamic stall/spin phenomena, both government and industry have sought remedial measures lessening pilot burden during operation of an aircraft under conditions close to stall speed.
In current general practice, control asserted over aircraft during typically slower speed departure and landing maneuvers has been predicated upon readout information gleened from the airspeed indicator, the pilot being called upon to correlate a particular flying situation with aircraft manufacturer recommended airspeeds. Such recommended speeds are considered to vary with a variety of aspects including the gross weight of the aircraft, best rate and angle of climb on take-off, desired glide path, maneuvering on landing, angle of bank, flank setting final approach techniques and the like. As is apparent, this form of control direction during slower speed maneuvering is not entirely desirable. Looking momentarily to the general theory involved in considering such slower speed flight, recourse usually is made to the interrelationships of the coefficient of lift, C.sub.L, coefficient of drag, C.sub.D, and angle of attack AOA, in defining the performance of a given aircraft. The coefficient of lift is represented by the formula: ##EQU1## where L is lift, .rho. is air density, S is the area of the wing airfoil and V is the velocity or air speed at the airfoil in question.
Similarly, the coefficient of drag, C.sub.D, is represented by the formula: ##EQU2## where D, is drag. The angle of attack for any given aircraft is measured as the angle between relative wind and the chord of its wing. This chord, for each wing configuration or airfoil condition, is considered to extend from the forwardmost leading edge of the wing airfoil to the farthest extent of the trailing edge.
In practice, the coefficients of lift and drag are plotted or compared against angle of attack for a given airfoil to derive desired lift/drag conditions. Generally, such comparisons will reveal that the coefficient of lift increases with angle of attack to a maximum value or curve inflection point. As angle of attack is increased beyond that maximum value, however, incipient stall conditions are encountered wherein rapid increases in drag develop and the corresponding values for coefficient of lift drop rapidly. Accordingly, operation of an aircraft at a condition near stall during commencement and termination of a flight will be at an angle of attack within a selected region short of the inflection point of the coefficient of lift curve. In practice, about a thirty percent factor of safety is recommended for such operation.
As is apparent, a continuous informational input to the pilot as to the value of actual angle of attack compared with the value thereof at a stall condition is of considerable control value. In this regard, techniques for deriving such informational inputs have been derived. For instance, a vane has been attached to the wing or fuselage of an aircraft to generate a continuous angle of attack information signal. However, the treatment of such signals to derive a valid relationship between vane-measured angle of attack and the corresponding angle at a stall condition necessarily is complex. Any alteration in the airfoil configuration for instance, as is occasioned with various degrees of flap deflection or, perhaps through the assertion of parasitic drags with the lowering of landing gear and the like, alters the chord orientation defining angle of attack and/or alter the coefficient of lift characteristic of the airfoil. In consequence, determination of a revised stall angle of attack is necessitated with each slight change of airfoil configuration. Systems have been evolved which accommodate to this variable through the use of on-board computers which correlate flap position and the like with apparent angle of attack to compute stall angle of attack. However, systems generally are too elaborate for utilization at the levels of aircraft expenditure found in most general aviation aircraft.
Another form of sensor which avoids the need for a combination of vane and computer to provide stall angle of attack information is conventionally referred to as a "lift transducer" and is described, inter alia, in U.S. Pat. Nos. 3,486,722; 3,437,292; and 3,361,393. The lift transducer is described as measuring the relative location of the airflow stagnation point at the leading edge of a wing airfoil. For instance, as such leading edge addresses an airflow i.e., relative wind, certain of that flow will go over and certain of the flow will go under the wing to form a pressure area defined as the noted stagnation point. As the angle of attack of the airfoil changes through the introduction of flaps or the like as well as changing aircraft attitude, this stagnation point moves correspondingly either forward or aft on the leading edge of the wing. By measuring such relative movement with respect to a given location, signal values corresponding with actual and stall angles of attack can be derived.
As is apparent, each of the disclosed systems seeks to generate an aerodynamic flight condition signal which, in effect, is a function of the instantaneous value of angle of attack or coefficient of lift and supplies some form of perceptible readout to the pilot during lower speed flight.
From the foregoing, it may be observed that a desirable lift director system for aircraft will provide information to a pilot as to optimum aircraft attitude during those lower speed maneuvers wherein the aircraft is operated near to a stall condition. This information is desired for achieving best rate or angle of climb following departure as well as during landing approaches. In the latter regard, such information desirably will permit approaches to be carried out under short field conditions as well as avoid stall conditions during directional maneuvers such as the turn from downwind or base leg.
Another aspect of such lift control is concerned with the final stage of descent for landing wherein the aircraft is "flared-out" to progressively reach a full-stall attitude at the point of touchdown. During this maneuver, the pilot is called upon to introduce a progressive variation of aircraft attitude in order to gradually reach the above noted full-stall attitude. This maneuver has been observed to be one of particular difficulty on the part of novice or beginning pilots. A director system assuring optimum transition during this phase in addition to the above will serve to significantly enhance both the quality and safety of landings. Similar difficulties have been witnessed during a corresponding phase of flight wherein the aircraft reaches the velocity for rotation during take-off role. Hereagain the pilot is called upon to commence a maneuver wherein an important progressive alteration of attitude of the aircraft is inserted. It has been observed that pilots in training or novice pilots experience some difficulty in achieving appropriate progressive attitude control during such maneuvers.
As noted above, during the critical, terminal phases of a flight the frequency of information supplied the pilot is increased to a relatively high level. Accordingly, where possible, such information should be asserted in as high an assimilatory fashion as possible to avoid the danger of confusion.